SEGMENT INFORMATION (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment information
Number of operating segments | segment									1
Disaggregation of revenue	$ 73,962	$ 80,092	$ 79,981	$ 76,576	$ 65,355	$ 64,968	$ 60,335	$ 61,354	$ 310,611	$ 252,012	$ 245,893
Identifiable assets	285,301				187,922				$ 285,301	$ 187,922
Total Revenue
Segment information
Percentage of concentration risk									19.00%	18.00%	19.00%
Trade receivables
Segment information
Percentage of concentration risk									13.00%	15.00%
Outside the United States
Segment information
Disaggregation of revenue									$ 146,835	$ 119,212	$ 111,993
Identifiable assets	$ 88,400				$ 84,652				88,400	84,652
Wholly owned foreign subsidiaries
Segment information
Disaggregation of revenue									$ 126,104	$ 107,039	$ 99,061